Switched-mode power converters are widely used to convert between differing DC voltages. A typical example of a buck converter (used to provide a reduced voltage from a higher-voltage supply) is shown in FIG. 1.
Transistor switches are used to alternately connect an inductor to a supply voltage and ground, at a switching frequency, fsw. The output of the inductor is connected to a load.
In some applications, the target output voltage of a given converter is fixed during its useful life. In other applications, the target output voltage may be changed. For example, in the case where a DC-DC converter is employed to provide power for the output amplifier of a wireless transmitter, it is well-known that improved overall system efficiency can be obtained if the converter output voltage is varied depending on the radio frequency power to be transmitted. The benefits of this procedure vary widely depending on the nature of the wireless signal to be transmitted. For example, signals used in cellular communications based on code-division multiple access (CDMA) use intentional control of average transmit power at each mobile station to ensure roughly equal received power at the basestation. In order to ensure this result, the transmitted power in a mobile station (a phone, handheld device, or data modem) is adjusted periodically. In many standards, this adjustment takes place at the beginning of a transmission “slot”, a fixed time period in which a fixed number of symbols are sent. For example, in WCDMA continuous transmission, average transmit power is changed at the beginning of each 667-microsecond slot. In order to optimize overall system efficiency, the supply voltage delivered to the transmit power amplifier may be similarly adjusted at the beginning of each slot. This adjustment may be accomplished using a linear regulator, but better system efficiency can be obtained with a switched-mode converter.
In modern wireless standards in which multiple streams of data are simultaneously sent using either code-division or orthogonal frequency division multiplexing (OFDM), the instantaneous amplitude of the transmitted signal varies considerably from one symbol to the next. Further efficiency improvements can be obtained if the supply voltage is similarly adjusted on a symbol-by-symbol basis; this mode of operation is known as Envelope Tracking. Envelope Tracking requires very rapid adjustments in the power amplifier supply voltage; in the WCDMA standard, the symbol duration is (1/3.84) microseconds, and the envelope of each symbol may vary in an uncorrelated, pseudo-random fashion when multiple coded streams are simultaneously transmitted. Undue delay or tracking errors in the supply may lead to distorted symbols, resulting in spurious output frequencies, and increases in the Error Vector Magnitude (EVM) of the transmitted signal. Envelope Tracking of such high-speed signals has usually been performed using linear regulators, or a linear regulator in combination with a switched-mode converter, because low-switching-frequency converters cannot provide the rapid response required for Envelope Tracking applications.
In the case where a power-controlled mobile station is reasonably dose to a basestation, very small transmit power may be sufficient to provide low bit error rates while minimizing interference. For example, it is known that when voice is being transmitted, a CDMA or WCDMA mobile station transmit power is most often adjusted to less than 10 mW, and frequently less than 1 mW, with only rare excursions to transmitted power greater than 100 mW. When the transmitted power is small, envelope tracking provides minimal benefits in total power consumed. However, switched converters are very inefficient at low power levels because of the substantial fixed overhead of switching power and controller power. It is well-known that substantial improvements in converter efficiency may be obtained in this case by making the switching transistors inactive for a period of time, allowing the load to discharge a storage capacitor until the output voltage drops to a voltage below the minimum desired. This form of operation is variously known as hysteretic control, pulse skipping, burst mode, or pulse frequency modulation.
It is desirable to have methods and apparatuses for voltage regulation that provides both high bandwidth/high power and low bandwidth/low power regulated voltages.